When Wal-Mart Stores does a thing, it does it big. The stock buy-back it announced at its annual meeting today is no exception, at $15 billion, hot on the heels of an earlier $15-billion repurchase plan.

So it’s probably a good time to remember that stock buybacks generally aren’t the unparalleled good that they can seem at first glance.

Investors can complain as loudly and clearly as they like, but corporate boards are often free to ignore them, with few or no immediate consequences. That’s true whether the protest involves ousting a board member or changing how the company does business.